BATTERY HYBRIDISATION
WIND ABOVE: WHAT GOES ON BELOW? Could wind’s promised efficiency impact other systems? Stevie Knight asks the questions...
8 Maersk Pelican’s engines absorbed the variable loads from the Flettner tow effect
Currently, “Flettner rotors or kites are rather easy to integrate” says Philip Holt of MAN ES. That’s partly because the effective power provided by these systems has historically been quite low: “We have heard of fuel saving of around 3.5% to 5% and up to 10% in special cases on merchant vessels”, he explains. That puts it in the same ballpark “as sailing in small waves with a tail wind.... for the same rpm setting, the propeller simply requires a lower amount of torque”. However, if the whispers of a near-future 20% and potential 40% vessel efficiency are to be believed, there are further considerations. “As these mechanical sails push the ship forward through the hull, the amount of additional energy introduced could vary a lot, from zero to half the installed power,” says Tuomas Riski of Norsepower, adding that the onboard plant needs to be flexible enough to compensate for these reduced loads. However, given fairly steady conditions, this still falls within the efficiency range for long-haul ships’ two-stroke engines. “If you’re operating on 50% engine load with the wind propulsion aid, and 90% without it, the specific fuel oil consumption (SFOC) will only deviate by a few grams,” says Holt, adding that MAN’s slow speed, two-stroke models tend to be efficient down to 10% MCR. It’s plausible that these engines’ load response might be a little slow, but as Van der Kolk goes on to explain, while most studies look at retaining a fixed speed, in fact there is a wider operating envelope as in reality some wind variation will simply be taken up by a greater ship speed. MARIN’s Rogier Eggers adds that even if a conventional, two-stroke propulsion plant isn’t fully adjusted for WASP, “the savings are likely to still be significant, even if they could be higher”. Riski remarks that experience bears this out: Maersk Pelican’s impressive 8.2%, savings resulted from installing a pair of large, 30m Rotor Sails without paying the engines any special attention. The 61,000gt tanker “has faced variable wind conditions on continuous basis, but” he stresses “it has not had any challenges in adapting to fluctuating loads”.
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However, a greater degree of wind assistance may change the picture: “We will have to see what is really possible with adjusting two-stroke propulsion,” says Eggers, though Holt adds that while “it’s mostly speculative”, during high levels of wind assistance, the engine might benefit from alternative optimisation parameters - although “firm consideration” is still over the horizon, “actual numbers will be required to perform a study of the potential”. Despite this, Holt underlines that even given sizeable WASP capacity, MAN ES “doesn’t recommend reducing the main engine power based on an installation of some sort of wind assistance... as it cannot be ensured to operate in heavy weather”. Still, WASP installation could give rise to alternative power configurations. Installing a shaft-generator PTO along with the WASP on deck “could be beneficial” says Holt, as the main engine may be used to supply the electrical power rather than the auxiliaries. It would make for an interesting set up, says Sami Kanerva, ABB Marine and Ports: the gensets could take a break as the two-stroke engine picks up the load during periods of greater wind assistance. NOT ONLY FOR THE BIG BOYS Ship type does play a part says Eggers: higher speed results in “the effective wind direction above deck being mostly from ahead”, so don’t expect long-haul containerships to line up for WASP installation, even if they could arrange it without sacrificing capacity. However, more modest general cargo vessels and coasters “are interesting, because their lower speed translates to a more favourable wind angle”, he remarks. And in this arena, four-stroke engines reach a broader audience. As Kanerva explains “in contrast to the larger oceangoing cargo ships, diesel-electric propulsion is typically applied to smaller vessels with a wider operating profile”, given this, it is “usually more effective to have multiple medium-speed engines”. This, of course, applies doubly to those looking at low-carbon or coastal emission-mitigation strategies.
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